# CVE-2026-77803: Front-end Desynchronization Vulnerability in Progress® Telerik® Fiddler® Classic

> A local attacker can exploit HTTP request desynchronization in Fiddler Classic to split requests and smuggle responses, bypassing security controls.

- **Severity:** Low 3.6 (`CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 5 Oct 2026
- **Updated:** 6 Oct 2026
- **Also known as:** GHSA-xq67-hr2w-mm32

## What is CVE-2026-77803?

CVE-2026-77803 is a low-severity vulnerability in Progress® Telerik® Fiddler® Classic, published on 5 Oct 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In Progress® Telerik® Fiddler® Classic for Windows, versions prior to v6.0.20262.10021, front-end request desynchronization is possible in the proxy request forwarding component. A request that contains both a Content-Length and a Transfer-Encoding header is forwarded with both headers present, while Fiddler frames the body using Transfer-Encoding only. The remaining bytes on the reused client connection are then parsed as a separate pipelined request, so a local threat actor with low privileges can cause a single malformed request to be split into two requests forwarded to the origin server and receive an additional smuggled response, without requiring a vulnerable server.

Weakness: CWE-444 (Inconsistent Interpretation of HTTP Requests)

## Which versions are affected by CVE-2026-77803?


## How do I fix CVE-2026-77803?

No fix has been published for CVE-2026-77803 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-77803 being exploited?

There is no known exploitation of CVE-2026-77803 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-77803 and fix it if it is affected.

What it is: CVE-2026-77803 is a low-severity vulnerability in Progress® Telerik® Fiddler® Classic, published on 5 Oct 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-77803 in the wild and no public exploit code.

Affected product: Progress® Telerik® Fiddler® Classic. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-77803/ (Markdown: https://insights.sbomify.com/CVE-2026-77803/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-77803
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-77803
- OSV: https://osv.dev/vulnerability/CVE-2026-77803

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-77803/
Summaries are generated and may contain mistakes; the official description above is the record.